Accurate, continuous measurement of the level of contained materials is important to the efficiency and economy of many operations in the mining and manufacturing industries. Bins, silos, and hoppers (both surface and underground types in different shapes and sizes) hold or convey large quantities of materials, with occasionally up to several thousand tonnes capacity. The diameters of such bins may exceed 20 m and bin heights of up to 50 m are common. The measurement of the level of the material in such bins is one of the problems associated with their use.
A variety of commercial level gauges are available at present. These include gauges which use mechanical, electro-mechanical, 1sonar, microwave and optical techniques, pressure switch type gauges, and gauges which use gamma ray transmission or backscatter.
The available gauges are, however, subject to several disadvantages. The contact type devices are subject to mechanical damage, and some devices cannot stand the extreme environment inside industrial bins (dust, noise, mechanical vibration, and the like). Exposed moving parts tend to get clogged up with dust. Sonar level gauges fail to indicate true level due to reflections from thick clouds of dust common in industrial bins, and the build up of dust in the sound generator receiver system is another problem. The optical level gauges also fail due to the dust problem. Because of the large dimensions of bins, the gamma ray transmission type level gauges require strong radiation sources which are a potential health hazard. Moreover, the use of such sources restricts access by maintenance workers to the inside of the bins.
The use of neutron irradiation has been established as a promising technique for level gauging of liquids containing hydrogen by several workers in this field. For example, U.S. Pat. No. 2,378,219 discloses a neutron level gauge for determining the level of a liquid in a vessel. The apparatus disclosed in that specification consists of a fast neutron source and a thermal neutron detector. The device is placed on the outside wall of the vessel and the fast neutrons thermalized by the liquid reach the detector. The level of liquid is determined by moving the source-detector assembly vertically along the wall and noting the point at which a sudden change in the thermal neutron intensity is indicated by the detector.
From the work of S. Barnartt and K. H. Sun (see their paper in "Nucleonics", Vol. 13, May 1955), it was shown that a neutrol level gauge employing a fast neutron source and a thermal neutron detector could be used for the determination of levels of hydrogenous and other light liquids to an accuracy of .+-.2 mm. Barnartt and Sun also suggested that their gauge could be placed either outside the vessel on the wall or inside the liquid.
The specification of British Pat. No. 938,233 describes a method and apparatus for determining the quantity of liquid in a container employing an assembly of fast neutron sources and a thermal neutron detector. In this case, the parameter measured is the quantity of the liquid in the container and not the level. U.S. Pat. No. 3,716,711 describes an apparatus consisting of a fast neutron source and a thermal neutron detector with suitable shielding intended for measuring the water level in the compartment of sunken vessels. Another application of the neutron thermalising properties of hydrogenous liquids is disclosed in the specification of British Pat. No. 1,288,263. In that apparatus, a lengthy thermal neutron counter and a large number of neutron sources are employed. The detector-neutron source assembly is placed directly inside the liquid in a vessel or inside a stand-pipe placed in the liquid. It should be pointed out that such a device is economically impractical for use in large vessels because of the prohibitive length of the neutron detector and the high cost of a large number of neutron sources.
All the above neutron level gauges have severe disadvantages in meeting the needs of modern industry. Firstly, considerable modification of the instrumentation is needed for the devices to function as continuous level gauges in large bins. Secondly, they can only be used for level gauging of liquids or solids containing hydrogen or other neutron moderating elements, and this precludes the use of these level gauges in bins which handle, for example, dry materials devoid of hydrogen, beryllium, carbon and the like. Consequently, there is a need for a continuous level gauge for solids and liquids which may not contain hydrogen.
It is an object of the present invention to provide a continuous level gauge which is applicable to both solids and liquids, whether or not they contain a neutron moderating element. Another object of the invention is to provide a level gauge which is simple, reliable, easy to operate and repair, and free of hazards applicable to bins of surface and of underground type, irrespective of their sizes and wall thickness. A further object is to provide a level gauge which can produce control signals to other devices, and therefore be of great help in automating various handling operations, thus improving the efficiency and lowering the operating cost of the facility with which they are used.